The present invention relates to a method for heat treating a pipe with a double-pipe section and more particularly relates to a heat treatment method especially adapted to improve residual stresses of welded joints of a metal pipe comprising interconnected double-pipe and single-pipe sections.
Of various types of pipes used in nuclear power plants, heat power plants and chemical plants, there are pipes each comprising a single-pipe section 1 and a double-pipe section 2 as shown in FIGS. 1 and 2. FIG. 1 shows the construction in which a thermal sleeve 6 with a ring-shaped space 5 is disposed in a nozzle 4 of a pressure vessel 3 of a nuclear reactor. FIG. 2 shows the construction in which a first supply pipe 7 is joined to a second supply pipe 8 and a thermal sleeve 6 is disposed so as to retard the mixing of a liquid supplied from the first supply pipe 7 with a liquid supplied through the second supply pipe 8. In such pipes with the above-described construction, pipe sections must be welded at joints 9 because of the limitations from a fabrication standpoint. Residual tensile stresses occur adjacent to the welded joints 9 due to weld heat.
It is well known that with some metals such as austenitic stainless steel widely used in nuclear power plants and chemical plants, stress corrosion cracking likely occurs and propagates when tensile stresses coexist with corrosion factors. In order to solve such problem, there has been proposed a stress improvement method in which a pipe is subjected to induction heating while cooling water flows through the pipe so that a temperature difference between the inner and outer wall surfaces of the pipe causes thermal stresses in excess of a yield point, thereby causing residual compression stresses in the inner wall surface.
The above-described method may be applied to a straight or other pipe which has a simple configuration, but cannot be applied to pipes having complicated constructions as shown in FIGS. 1 and 2. If this method were employed to the latter pipes, the double-pipe section represented by point A would be excessively heated. There would arise a problem that a required temperature difference cannot be obtained if attempts were made to decrease the heat input for prevention of such excessive heating.
The present invention can effectively solve the problems encountered in the prior art and make it possible to change or convert residual tensile stresses in the inner wall surface of a welded joint to the residual compressive stresses, thereby suppressing the occurence and propagation of stress corrosion cracking. The present invention also provides a heat treatment method which can be carried out in a very simple manner.
The inventor found out that since the dead water or liquid removes the heat from the double-pipe section only by natural convection, the cooling capability thereat is very small in the heating process. In view of this fact, the present invention provides a heat treatment method in which a pipe is brought into contact with flowing water and dead water; a single-pipe secton is heated so that a temperature difference causes thermal stresses in excess of a yield point in the wall of the single-pipe section; thereafter the pipe is heated for a short time so that thermal stresses in excess of a yield point are produced adjacent to the wall surface of a double-pipe section; and after some time period, the heating of the single-pipe section is stopped.
The above and other objects, effects, features and advantages of the present invention will become more apparent from the following description of a preferred embodiment thereof taken in conjunction with the accompanying drawings.